Liposomes are closed vesicles of bimolecular layer (liposomal membrane) formed mainly of phospholipid and having a structure and functions similar to living membrane, which have been employed as various pharmaceutical materials. Liposomes can construct a so-called capsule structure in which water-soluble chemical substances are included in the internal aqueous phase and oil-soluble chemical substances are retained in the interior of the bimolecular layer and have been employed in various fields such as medical diagnosis, medical treatments and cosmetics. Recently, active studies have been made studies of application to a drug delivery system (DDS).
Hitherto, there has been employed the Bangham method or the reverse phase vaporization method (also denoted as the REV method) to prepare drug-enclosing liposomes. In these methods, drugs are enclosed within liposomes exhibiting high safety as raw materials and appropriate degradability in vivo, whereas it is necessary to use organic solvents as a solvent for phospholipid forming the liposomal membrane in the process of preparing liposomes. However, the thus obtained liposome-containing pharmaceutical preparation results in problems that an unremoved organic solvent remains in the preparation, exerting a toxic effect upon the living body.
Further, conventional methods are difficult to allow a sufficient amount of drugs to be enclosed in the liposomes so that problems arise which necessitate dosing of relatively large amounts of the liposome-containing pharmaceutical preparation, imposing an excessive burden on the patient. Taking into account application to contrast medium for diagnosis of which dose becomes large, compared to drugs for medical treatments, there has been desired a liposome-containing pharmaceutical preparation exhibiting high retention efficiency (enclosure ratio) of contrast medium material.
JP-A No. 2003-119120 (hereinafter, the term JP-A refers to Japanese Patent Application Publication) discloses a method of manufacturing liposomes enclosing water-soluble electrolytic chemicals, using supercritical carbon dioxide. This method which is feasible to set various manufacturing conditions can achieve enhanced enclosure of water-soluble chemicals more easily than the conventional method of manufacturing a liposome suspension. However, the use of supercritical carbon dioxide, as disclosed in the foregoing patent document enables preparation of liposomes having enhanced enclosure of water-soluble chemicals, compared to the conventional methods but still further enhancement of the enclosure ratio has been desired.
When liposomes applied to the uses including diagnosis and medical treatments are unstable in vivo, the liposomal membrane is ruptured and enclosed chemicals are effused in an early stage. As a result, there occurred problems that the chemicals were not delivered to the intended site and it was difficult to control sustained-release as one of the functions of the liposome. Accordingly, liposomes exhibiting superior stability in vivo have been desired.
To solve the foregoing problems, there has been disclosed a method of allowing a buffer solution exhibiting a pH of 9.5 or less at 15° C. and containing ammonia or a water-soluble amine to be included in the liposome dispersion to enhance stability of the liposome, as described in published Japanese translation of PCT international publication for patent applications. However, even liposomes obtained by such a method still had a problem with respect to stability in vivo.